464 research outputs found
Axions and their Distribution in Galactic Halos
Axion physics is briefly reviewed. Constraints from laboratory searches,
astrophysics and cosmology require the axion mass to be in the range eV. Near the lower end of this range, axions are
all or a major component of the cold dark matter of the universe. The late
infall of axions, and of any other cold dark matter particles, onto our galaxy
produces streams and caustics in its halo. The outer caustics are topological
spheres whereas the inner caustics are rings. The self-similar model of
galactic halo formation predicts that the caustic ring radii obey the
approximate law . Evidence for this law has been found in a
statistical study of 32 extended and well-measured external galactic rotation
curves, and in the existence and distribution of sharp rises in the Milky Way
rotation curve. Moreover, a triangular feature in the IRAS map of the Galactic
plane is consistent with the imprint of a ring caustic upon the baryonic
matter. Its position coincides with a rise in the rotation curve, the one
nearest to us. These observations imply that the dark matter in our
neighborhood is dominated by a single flow. Estimates of that flow's density
and velocity vector are given.Comment: 10 pages, 1 figure, invited talk at the IDM2002 conference, September
2-6, 2002 in York, Englan
Axion detection in the milli-eV mass range
We propose an experimental scheme to search for galactic halo axions with
mass eV, which is above the range accessible with cavity
techniques. The detector consists of a large number of parallel superconducting
wires embedded in a material transparent to microwave radiation. The wires
carry a current configuration which produces a static, inhomogeneous magnetic
field within the detector volume. Axions which enter this
volume may convert to photons. We discuss the feasibility of the detector and
its sensitivity.Comment: LaTex, 9 pages, 4 figures (sent upon request), UFIFT-HEP-93--
Evidence for universal structure in galactic halos
The late infall of dark matter onto a galaxy produces structure (such as
caustics) in the distribution of dark matter in the halo. We argue that such
structure is likely to occur generically on length scales proportional to , where is the age of the universe and is the
rotation velocity of the galaxy. A set of 32 extended galactic rotation curves
is analyzed. For each curve, the radial coordinate is rescaled according to
, where we choose . A
linear fit to each rescaled rotation curve is subtracted, and the residuals are
binned and averaged. The sample shows significant features near and . This is consistent with the predictions of the
self-similar caustic ring model of galactic halos.Comment: 4 pages, LaTeX, 1 epsf figur
Gravitational Lensing by Dark Matter Caustics
Dark matter caustics have specific density profiles and, therefore, precisely
calculable gravitational lensing properties. We present a formalism which
simplifies the relevant calculations, and apply it to four specific cases. In
the first three, the line of sight is tangent to a smooth caustic surface. The
curvature of the surface at the tangent point is positive, negative or zero. In
the fourth case the line of sight passes near a cusp. For each we derive the
map between the image and source planes. In some cases, a point source has
multiple images and experiences infinite magnification when the images merge.
Unfortunately, for the dark matter caustics expected in realistic galactic halo
models, the angular resolution required to resolve the multiple images is not
presently achievable. A more promising approach aims to observe the distortions
caused by dark matter caustics in the images of extended sources such as radio
jets.Comment: 36 pages, 11 figure
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